FIG. 1 illustrates an aircraft powered by counterrotating propellers 6 and 9 of the ultra-high bypass, unducted fan type. The rotational directions are indicated by arrows 12 and 15. FIG. 2 illustrates a type of turbine system which can be used to directly drive the propellers 6 and 9.
In FIG. 2, fore propeller 9 (hatched) is attached to a first turbine 18 (also hatched) which rotates in direction 15 as indicated in FIG. 1. The aft propeller 6 is attached to a second turbine 21 and rotates in direction 12 of FIG. 1. Bearings 140 support the turbines and allow the rotation. A hot, high energy gas stream 30 in FIG. 2 is provided by a gas generator (not shown) and causes rotation of the turbines.
Propeller blades 6 and 9 (which are sometimes called fan blades, prop fans, or propulsor blades in the art because they have characteristics of hybrids between propellers and fans) are of variable pitch type. Variable pitch means that each blade can rotate about a respective pitch axis 6A or 9A in FIG. 2, as indicated by circular arrows 33. The principle reason for changing the pitch is to provide the blades with the angle of attack which is proper under the prevailing flight conditions of the aircraft and power setting of the engine.
The propeller blades can be carried by an annular carrier, such as the polygonal ring 24 shown in FIG. 3. The polygonal ring surrounds the turbine stage 23, which is shown in schematic cross-section in FIG. 2.
The source of motive power for causing the pitch change is generally located inside the annular flowpath of turbines 18 and 21 which gas stream 30 follows, such as in region 35 in FIGS. 2 and 3. Consequently, a mechanism is needed to carry mechanical torque from region 35, across the turbine flowpath 30, to the blades 6 and 9.